Hi, and welcome to OBiLand.
Obihai Technology was bought by Polycom, which was then bought by Plantronics. The three companies separately made/make complementary equipment, and together, they can offer complete solutions to business clients and resellers. The OBiTALK Analog Telephone Adapters (OBi 200 and 500 series) and the OBiTALK IP phones (2000 series) will continue to be sold via retail channels (e.g. Amazon, Newegg, B&H Photo...). These devices will continue to support Google Voice and other SIP ITSPs. The Polycom-heritage IP phones will continue to be sold to distributors/VARs/integrators and to large enterprise customers, and will not support the consumer version of Google Voice.
So: I wouldn't worry about it. Of course, the usual disclaimer applies: the marketplace is always changing, and there is no guarantee of future support.
An OBi 200 or 202 are ATAs that support one or two ordinary analog telephones with RJ-11 phone jacks. Any typical residential corded or cordless phone should work (of course, we are talking about TouchTone phones, not rotary dial phones). Many users simply plug one DECT 6.0 cordless phone base station into the OBi device, and then scatter cordless handsets around the premises.
The OBi 508 is not necessary for your use case. You can either use cordless handsets per above, or you can connect the OBiTALK device's phone jack to your premises telephone wiring, and then plug in corded phones to phone jacks in various rooms. However, you absolutely must first physically disconnect the wires from the telephone company that were/are still connected to your building, even if you no longer subscribe to their service, or else you risk frying the OBi.
https://www.amazon.com/gp/product/B00BUV7C9A/No, the OBi ATAs are
ANALOG telephone adapters. You plug ordinary analog telephones into them. IP phones do not plug into OBi devices. They are all digital (VoIP) up to the point that the signals are converted to analog sound for the phone's mic and speaker/handset.
If you want IP phones that work with Google Voice, then by the OBi 2182 phones.
https://www.amazon.com/Obihai-OBi2000-Gigabit-Phones-OBi2182/dp/B076JKV5CL